Generally, CdTe single crystals having high purity and high resistance are suitable for substrates for semiconductor devices, such as radiation detector, IC tester and the like. It is known that the higher the resistance of the CdTe single crystal to be used is (for example, not less than 1.0×109 Ω·cm), the more the device characteristic improves.
In earlier technology, for producing a Group II–VI compound semiconductor single crystal, such as CdTe or the like, the THM (traveling heater method) of putting a columnar raw material having the same composition as the growing compound crystal and a material to be a solvent for dissolving the raw material at the time of crystal growth into a growth container in a state that the raw material is placed on the solvent material, heating locally to melt the solvent with a heater and forming a melting zone by also melting the lower end of the raw material, and thereafter, depositing continuously to grow a single crystal from the lower end of the melting zone by moving the heater upward or by moving the growth container downward has been generally used. It is said that the THM makes it possible to grow a Group II–VI compound semiconductor single crystal having high purity and high resistance more than other methods, such as vertical gradient freezing (VGF) method and the like.
Further, in order to make the resistance of a CdTe single crystal high, for example, a method of adding halogen, such as chlorine or the like, in the crystal has been used. However, when the amount of chlorine for adding into the crystal is increased in order to make the resistance of the CdTe single crystal become high resistance, the lifetime of the carrier (electron or electron hole) generated by inputting radiation becomes small. Thereby, there is a problem such that the sensitivity of a radiation detector becomes low. Therefore, a technique for producing a CdTe single crystal whose chlorine concentration is low (not more than 5 ppmwt) and having high resistance (not less than 1.0×109 Ω·cm) has been required.
In order to achieve the above-described object, for example, a technique such that the resistance of a CdTe single crystal is made high by heat-treating a CdTe crystal at a temperature between 350° C. and 450° C. after growing the CdTe crystal in which chlorine is doped by utilizing the THM is disclosed (Japanese Patent Laid-Open Publication No. 5-283729).
Further, since a semiconductor single crystal having a size not less than several mm square is required for producing a semiconductor device, such as a radiation detector or the like, a technique such that a CdTe single crystal having a large particle diameter is grown by utilizing the THM is disclosed (Japanese Patent Laid-Open Publication No. 7-300387 and Japanese Patent No. 2844430).
However, with the technique in the Japanese Patent Laid-Open Publication No. 5-283729, a CdTe single crystal having high resistance can be produced compared with the producing methods of utilizing the THM in the earlier technology, however, it is unable to realize a CdTe single crystal whose chlorine concentration is not more than 5 ppmwt and resistivity not less than 1.0×109 Ω·cm cannot be realized. Therefore, it cannot say that the above-described CdTe single crystal is most suitable as a semiconductor single crystal for using as a substrate for a semiconductor device, such as a radiation detector or the like, and there is room for improvement.
On the other hand, in the technique of the Japanese Patent Laid-Open Publication No. 7-300387 or Japanese Patent No. 2844430, a technique of attempting to make the particle diameter of the crystal large is disclosed. However, the size of the obtained single crystal is 30 mm at most.
Further, generally, in the growth of single crystal according to the THM, the growth rate is extremely slow as approximately 5 mm per day, so that there is a disadvantage that its production efficiency is bad compared with the VGF method or the like.
Thus, with the methods for producing a CdTe single crystal by the THM in the earlier technology, it is difficult to produce a CdTe single crystal having high purity and high resistance with sufficient productivity.
An object of the present invention is to provide a CdTe single crystal having high purity and high resistance (not less than 1.0×109 Ω·cm) and a CdTe polycrystal which is most suitable as a raw material of the above-described CdTe single crystal, and to provide a method for producing a CdTe single crystal excellent in productivity.